Marine and other aquatic sediments containing labile or reactive organic matter such as sewage, fish wastes, fish food and algal detritus in amounts over about 3% by weight total organic carbon (TOC) typically have a high sediment oxygen demand (SOD). This is due to the high rate of oxidation of such organic matter. In aquatic environments that receive high input rates of such reactive organic matter, SOD builds up over time until the oxygen demand is much greater then the supply of oxygen from the overlying water column. When oxygen demand exceeds supply, the overlying water column can be depleted of sufficient oxygen to support higher forms of aquatic life such as edible crustaceans and fish. It is, therefore, most important that the earliest stages of elevated SOD be predicted or detected to locate those areas of the sea floor that are potential candidates for developing hypoxic bottom water. Such prediction or detection will provide an opportunity to manage or otherwise control sources of organic enrichment before the environment becomes inhospitable to aquatic life.
Direct monitoring of dissolved oxygen (DO) in near-bottom marine sediment has been the traditional approach used to assess oxygen enrichment or depletion conditions in overlying water columns. Heretofore, it has been proposed to directly monitor DO using (1) chemical methods such as Winkler titration, (2) electro-optical methods such as fiber optic chemical sensors, and (3) polarographic electrodes.
Winkler titration requires that a small volume of water be recovered from a body of water. The sampling process must be carefully controlled so that the sample is not exposed to contamination by other sources of oxygen. The recovered sample is placed into glass bottles and the inventory of oxygen fixed with a reagent. The weight or volume of oxygen is measured by calorometric titration and expressed as the weight or volume of oxygen per unit volume of water. The Winkler titration method is limited to water column measurements and is not amenable to long time-series measurements as discrete water samples must be taken, usually by hand. The expense of taking time-series samples of water for titration analysis over extended periods of time makes this technique inappropriate for extended sampling in the field.
Fiber optical chemical sensors have been developed for monitoring dissolved oxygen in blood and appear to have potential for monitoring dissolved oxygen in aquatic systems. While such optical fibers may be insensitive to chemical poisoning they appear to be vulnerable to biofouling.
Polarographic membrane/electrode measurements of dissolved oxygen are among the more widely used means for measuring either the partial pressure of oxygen (volume or weight of oxygen per unit volume of water) or the flux of electrons (the oxidation-reduction "redox" potential E.sub.h, expressed as millivolts). Such instruments may be lowered into the water or sediment columns for the purpose of making instantaneous measurements of DO concentration or E.sub.h values immediately around the probe. Alternatively, such polarographic devices may be incorporated into chambers placed on the sea bottom for the purpose of making time-series measurements of water trapped inside a respiration chamber. The chamber is closed on all sides except the bottom and is carefully placed onto the sediment surface. The open bottom is then gently forced a few centimeters into the sediment layer in order to prevent exchange of the water trapped inside the chamber with outside ambient water. Such a method is used to measure the uptake or release of oxygen into the supernatant water above the bottom in the chamber over time periods ranging from a few minutes to a few hours. The resulting time-series measurements are used to calculate SOD expressed either as weight or volume of oxygen consumed or released from the sediments per unit volume of water per unit time per unit area of bottom. Such polarographic electrode measurements of dissolved oxygen are limited to water column measurements. Also, the length of deployment is limited by fouling of the external membrane surface of the electrode by marine growth and the poisoning of the electrode by hydrogen sulfide in the water. Further, when polarographic electrodes are placed in respiration chambers, the measured values of oxygen consumption are sensitive to the rate of stirring of the water. These measurements are short term (a few hours) and not amenable for routine long-term monitoring as the chamber inhibits the supply of oxygen to the area of bottom being measured.
In use, it is found that all of the foregoing methods are subject to wide temporal and spatial variations. In many cases, by the time low oxygen partial pressures in the overlying water are detected and measured, mass mortalities of aquatic fauna have already taken place. Furthermore, with existing polarographic techniques, it is difficult to measure DO gradients within laminar sublayers to detect the early onset of hypoxia. This is mainly because of the large bulk dimensions of currently available off-the-shelf polarographic DO probes. While smaller research polarographic probes have been developed to profile DO on a millimeter-by-millimeter scale, they are very fragile and not suited for rapid and efficient routine surveying of the sea floor.
Accordingly, there are continuing needs for improved apparatus and methods which simply and reliably allow (1) time-integrated measurement of the organic loading of a sea floor well in advance of the onset of bottom water hypoxia and (2) the making of numerous measurements over large areas in a survey day to locate and map zones of impending high surface SOD. The present invention satisfies such needs by use of in situ magnetometry to predict the onset of high SOD and water column hypoxia before biological resources are suffocated.